As per the Gtk signal handlers, there's no “user data” parameter to
proc. proc is called (proc handlername object
signalname signaldata connectobject after?).

If proc is not given, or is #f, the default is to connect
signalname on connectobject (or object if no
connectobject) to the procedure handlername. handlername is
evaluated as Scheme code, and so can simply be the name of a Scheme procedure,
or it can be a lambda form or similar evaluating to a procedure.